An ever-increasing number of relatively inexpensive, low power wireless data communication services, networks and devices have been made available over the past number of years, promising near wire speed transmission and reliability. Various wireless technology is described in detail in the 802 IEEE Standards, including for example, the IEEE Standard 802.11a (1999) and its updates and amendments, the IEEE Standard 802.11g (2003), and the IEEE Standard 802.11n now in the process of being adopted, all of which are collectively incorporated herein fully by reference. These standards have been or are in the process of being commercialized with the promise of 54 Mbps or higher data rate, making them a strong competitor to traditional wired Ethernet and the more common “802.11b” or “WiFi” 11 Mbps mobile wireless transmission standard.
Generally speaking, transmission systems compliant with the IEEE 802.11a and 802.11g or “802.11a/g” as well as the 802.11n standards achieve their high data transmission rates using Orthogonal Frequency Division Multiplexing (OFDM) encoded symbols mapped up to a 64 quadrature amplitude modulation (QAM) multi-carrier constellation. Generally speaking, the use of OFDM divides the overall system bandwidth into a number of frequency sub-bands or channels, with each frequency sub-band being associated with a respective subcarrier upon which data may be modulated. Thus, each frequency sub-band of the OFDM system may be viewed as an independent transmission channel within which to send data, thereby increasing the overall throughput or transmission rate of the communication system.
Generally, transmitters used in the wireless communication systems that are compliant with the aforementioned IEEE 802.11a/802.11g/802.11n standards as well as other standards such as the IEEE 802.16 Standard, perform multi-carrier OFDM symbol encoding (which may include error correction encoding and interleaving), convert the encoded symbols into the time domain using Inverse Fast Fourier Transform (IFFT) techniques, and perform digital to analog conversion and conventional radio frequency (RF) upconversion on the signals. These transmitters then transmit the modulated and upconverted signals after appropriate power amplification to one or more receivers, resulting in a relatively high-speed time domain signal with a large peak-to-average ratio (PAR).
Likewise, the receivers used in the wireless communication systems that are compliant with the aforementioned IEEE 802.11a/802.11g/802.11n and 802.16 standards generally include an RF receiving unit that performs RF downconversion and filtering of the received signals (which may be performed in one or more stages), and a baseband processor unit that processes the OFDM encoded symbols bearing the data of interest. Generally, the digital form of each OFDM symbol presented in the frequency domain is recovered after baseband downconversion, conventional analog to digital conversion and Fast Fourier Transformation of the received time domain analog signal. Thereafter, the baseband processor performs frequency domain equalization (FEQ) and demodulation to recover the transmitted symbols, and these symbols are then processed in a Viterbi decoder to estimate or determine the most likely identity of the transmitted symbol. The recovered and recognized stream of symbols is then decoded, which may include deinterleaving and error correction using any of a number of known error correction techniques, to produce a set of recovered signals corresponding to the original signals transmitted by the transmitter.
To further increase the number of signals which may be propagated in the communication system and/or to compensate for deleterious effects associated with the various propagation paths, and to thereby improve transmission performance, it is known to use multiple transmit and receive antennas within a wireless transmission system. Such a system is commonly referred to as a multiple-input, multiple-output (MIMO) wireless transmission system and is specifically provided for within the IEEE 802.11n Standard. Further, the IEEE 802.16 Standard, or WiMAX, applies to cell-based systems and supports MIMO techniques. Generally speaking, the use of MIMO technology produces significant increases in spectral efficiency and link reliability of IEEE 802.xx and other systems, and these benefits generally increase as the number of transmit and receive antennas within the MIMO system increases.
In addition to the frequency channels created by the use of OFDM, a MIMO channel formed by the various transmit and receive antennas between a particular transmitter and a particular receiver includes a number of independent spatial channels. As is known, a wireless MIMO communication system can provide improved performance (e.g., increased transmission capacity) by utilizing the additional dimensionalities created by these spatial channels for the transmission of additional data. Of course, the spatial channels of a wideband MIMO system may experience different channel conditions (e.g., different fading and multi-path effects) across the overall system bandwidth and may therefore achieve different SNRs at different frequencies (i.e., at the different OFDM frequency sub-bands) of the overall system bandwidth. Consequently, the number of information bits per modulation symbol (i.e., the data rate) that may be transmitted using the different frequency sub-bands of each spatial channel for a particular level of performance may differ from frequency sub-band to frequency sub-band.
In all communication systems discussed above, receivers sometimes fail to successfully receive data due to noise, interference, temporary resource failure, or other reasons. Accordingly, communication protocols typically support one or several error control methods to detect and compensate for transmission errors. A receiver may, for example, confirm that a data packet has been received and successfully decoded by sending a short acknowledgement message back to the transmitter. Additionally, the transmitter may start a timer upon sending a data packet and consider the data packet lost or undelivered if the transmitter does not receive an acknowledgement from the receiver before the timer expires. Some protocols additionally specify negative acknowledgements (NACKs) for acknowledging the receipt of a data packet while indicating that the data packet includes errors, for example.
In response to determining that the receiver did not properly receive the data packet, the transmitter typically retransmits the data packet immediately or after a certain controlled interval. This technique, generally known as Automatic Repeat Request (ARQ), includes multiple variants that limit the number of retransmissions or associate acknowledgments with frames rather than data packets, for example. In any ARQ technique, the receiver discards the defective data packet and does not generate a positive acknowledgement until it receives an acceptable copy of the data packet.
On the other hand, Hybrid Automatic Repeat Request (HARQ) techniques allow receivers to combine several defective copies of a data packet in an attempt to generate an acceptable version of the data packet. To this end, some HARQ recombining techniques such as Incremental Redundancy (IR), for example, require attaching different parity bits to the data packet during each retransmission to improve the chances of successfully combining defective copies into an acceptable version. The HARQ recombining technique known as Chase Combining, on the other hand, requires only that the same copy of the data packet be-transmitted and accordingly demands little computational complexity in recombining copies at the receiver. Because of its simplicity, Chase Combining HARQ is used in many systems such as WiMAX, Long Term Evolution (LTE), and other systems.
However, in OFDM and similar systems, Chase Combining HARQ frequently fails to generate an acceptable version of a transmitted data packet because interference or noise may corrupt the same bits in the data packet during each retransmission.